Two ancient hominid species, each with distinct walking patterns, crossed paths in East Africa. Footprints preserved on a former muddy lakeshore suggest that these species, adapted to walk in their own unique ways, coexisted around 1.5 million years ago. Newly discovered footprints at a site in northern Kenya, along with previously unearthed prints from a nearby location, provide evidence of potential interactions between ancient hominid species over a period of up to 200,000 years, according to paleoanthropologist Kevin Hatala of Chatham University in Pittsburgh and his colleagues.
The researchers report in the November 29 issue of Science that two distinct patterns of upright walking are evident in the foot tracks found along an ancient lake at Koobi Fora, a deposit on the eastern edge of present-day Lake Turkana. A similar distinction is noted in footprints excavated nearly 20 years ago at Ileret, another Kenyan site dating back to approximately 1.5 million years ago.
Hatala identifies prints showing human-like foot anatomy and gait as belonging to Homo erectus, a possible ancestor of Homo sapiens. H. erectus, which thrived from nearly 2 million to roughly 117,000 years ago, consumed a variety of energy-rich foods to support its large brain. Impressions displaying fewer similarities to modern human feet and walking patterns are believed to belong to Paranthropus boisei. This species, with its small brain and large jaw, lived between 2.3 million and 1.2 million years ago and had a diet rich in grasses and sedges.
Researchers have long known that East African fossils of H. erectus and P. boisei date to roughly the same period in nearby locations. However, the slow accumulation of these fossils made it difficult to determine if the two species coexisted in the same area. The preserved footprints analyzed in the new study resolve this issue, according to paleoanthropologist Jeremy DeSilva of Dartmouth College, who was not part of Hatala’s team. “We now know for certain that these two types of hominids shared the same landscape and walked with slightly different gaits.”
The closely spaced footprints at the new Koobi Fora site, consisting of three H. erectus impressions and a trail of 12 impressions left by a P. boisei individual, were formed and buried by lakeside sediments within a few days at most. This rapid burial also applies to footprints of large birds and animals such as antelopes and wild horses.
“Whether Homo and Paranthropus individuals passed through the area hours to a day apart, or seconds to a minute apart, they would have been aware of each other’s existence on this shared landscape,” Hatala says. Paleoanthropologist Bernard Wood of George Washington University suggests that if chimpanzees and gorillas can peacefully share a tree, it’s possible that H. erectus and P. boisei “met in a 1.5-million-year-old version of a 7-Eleven store” at a lake offering a variety of desirable foods.
While the footprint findings suggest interaction between H. erectus and P. boisei, “whether or when they competed, potentially due to climatic or environmental pressures, cannot be determined with the current evidence,” says paleoanthropologist Rita Sorrentino of the University of Bologna.
The Kenya footprints support a previous report of divergent upright stances among even older hominid species. At Tanzania’s Laetoli site, 3.6-million-year-old footprints include human-like impressions of Australopithecus afarensis and more chimp-like tracks of an unidentified hominid species.
In the new study, researchers compared digital 3-D models of ancient hominid footprints and trackways to those made by modern humans and chimps traversing muddy soil. Arches formed in human footprints when walking through mud resemble those left by H. erectus at the ancient lake, indicating that H. erectus moved its feet similarly to modern humans. P. boisei footprints displayed a flatter arch, showing differences in foot motion and anatomy compared to modern humans.
P. boisei also had big toes that splayed more than those of modern humans but less than chimps, suggesting greater mobility than in H. erectus or modern humans. These foot differences underlie two equally effective walking styles. “The trackway attributed to P. boisei reflects a fairly fast walking speed, and there is no evidence that they were off-balance or any less adept at walking on two legs than H. erectus,” Hatala concludes.
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